The present invention relates to a method for producing a grain-oriented silicon steel sheet, not having inorganic mineral films, by using an annealing separator capable of preventing inorganic mineral film composed of forsterite (Mg2SiO4), and so on, from forming during final annealing.
A grain-oriented silicon steel sheet is widely used as a material for magnetic cores and, for minimizing energy loss in particular, a silicon steel sheet having a small core loss has been sought. It is effective to impose a tension on a steel sheet for reducing a core loss. For this reason, it has been a common practice to create a tension and to reduce a core loss by forming coating films consisting of a material having a smaller thermal expansion coefficient than that of a steel sheet at a high temperature. A film of a forsterite type formed through the reaction of oxides on a steel sheet surface with an annealing separator in a final annealing process creates a tension in the steel sheet, and the adhesiveness of the film is excellent.
For example, the method for forming insulating coating films by coating the surfaces of a steel sheet with a coating liquid mainly consisting of colloidal silica and phosphate and by baking it, as disclosed in Japanese Unexamined Patent Publication No. S48-39338, has a significant effect on creating a tension in the steel sheet and is effective in reducing the core loss.
Therefore, the method of keeping the films of a forsterite type formed in a final annealing process and then forming insulating coating films mainly consisting of phosphate is generally employed as a method for producing a grain-oriented silicon steel sheet.
In recent years, it has been clarified that the disordered interfacial structure of a forsterite type film and a base metal deteriorates the effect of a film tension on improving a core loss to some extent. In view of this, a technology has been developed which attempts to further reduce a core loss by forming anew tension-creating coating films after removing the forsterite type films formed in a finish annealing process and/or further applying a mirror-finish, as disclosed in Japanese Unexamined Patent Publication No. S49-96920 for example.
However, the removal of a forsterite type coating film intruding into a steel sheet requires a labor. For instance, when the removal of the film by pickling is attempted, as forsterite contains a silica component, it is necessary to dip the film for a long time in a liquid of strong acid, such as hydrofluorid acid, capable of dissolving even a silica component. On the other hand, when the removal of the film by such a means as mechanical surface grinding is attempted, it is necessary to grind a steel sheet to a depth of nearly 10 xcexcm to completely remove the intruding portions of the film and thus the means is hardly acceptable from the viewpoint of the yield. What is more, the method of removing a film by grinding unavoidably introduces a strain in the steel sheet during the grinding work and causes deterioration of magnetic properties.
In consideration of the above, a technology of not forming inorganic mineral coating films composed of forsterite and so on during final annealing has been studied, rather than the technology of removing the forsterite films formed during a final annealing process after the annealing. During the course of the study, alumina attracted attention as an annealing separator with which oxides hardly remained after final annealing and, as a consequence, various technologies were disclosed in relation to an annealing separator consisting mainly of alumina.
For example, U.S. Pat. No. 3,785,882 discloses a method wherein alumina at 99% or more in purity and 100 to 400 mesh in grain size is used as an annealing separator, and Japanese Unexamined Patent Publication No. S56-65983 discloses another method wherein an annealing separator mainly composed of aluminum hydroxide is used in annealing. Besides these, Japanese Examined Patent Publication No. S48-19050 discloses a method wherein an annealing separator produced by adding an alkali metallic compound containing a boric acid component to alumina is used in annealing.
Further, Japanese Examined Patent Publication No. S56-3414 discloses a method wherein an annealing separator containing hydrous silicate powder by 5 to 40% with the balance consisting of alumina is used in annealing, and Japanese Examined Patent Publication No. S58-44152 discloses a technology wherein an annealing separator containing, in addition to hydrous silicate powder, a compound of strontium and/or barium by 0.2 to 20% and calcia and/or calcium hydroxide by 2 to 30% with the balance consisting of alumina is used in annealing.
More recently, Japanese Unexamined Patent Publication No. H7-18457 discloses a method wherein a mixture of coarse alumina with an average grain size of 1 to 50 xcexcm and fine alumina with an average grain size of 1 xcexcm or less is used as an annealing separator.
In many of the disclosed technologies wherein alumina is used as an annealing separator, the grain size of the alumina is prescribed.
In addition, Japanese Unexamined Patent Publication No. S59-96278 discloses a method wherein inert magnesia having a specific surface area of 0.5 to 10 m2/g, which is produced by calcining it at 1,300xc2x0 C. or higher and then crushing it, is added by 15 to 70 to alumina of 100 in terms of weight.
The effect of preventing the formation of a forsterite film can be obtained to some extent by employing any of the above methods and applying finish annealing to a steel sheet after it is subjected to decarburization annealing. However, it has been difficult to stably produce a final-annealed steel sheet on which neither forsterite films are formed nor oxides remain.
The present invention is a method of stably producing a final-annealed steel sheet on which neither forsterite films are formed nor oxides remain by solving the above problems, and the gist of the present invention is as follows:
(1) A method for producing a grain-oriented silicon steel sheet not having inorganic mineral coating films, comprising the steps of decarburization annealing, the coating an annealing separator and final annealing, wherein alumina powder calcined at a calcining temperature of 900 to 1,400xc2x0 C. is used as an annealing separator.
(2) A method for producing a grain-oriented silicon steel sheet not having inorganic mineral films comprising the steps of decarburization annealing followed by coating of annealing separator and final annealing, according to the item (1), wherein the alumina powder having a BET specific surface area of 1 to 100 m2/g is used as an annealing separator.
(3) A method for producing a grain-oriented silicon steel sheet not having inorganic mineral films comprising the steps of decarburization annealing followed by coating of annealing separator and final annealing, according to the item (1) or (2), wherein the alumina powder having an oil absorption of 1 to 70 ml/100 g is used as an annealing separator.
(4) A method for producing a grain-oriented silicon steel sheet not having inorganic mineral films comprising the steps of decarburization annealing followed by coating of annealing separator and final annealing, according to any one of the items (1) to (3), wherein the alumina powder having a xcex3 ratio of 0.001 to 2.0 is used as the annealing separator, where the xcex3 ratio is the ratio of the diffraction intensity from the (440) plane of a xcex3-alumina phase to the diffraction intensity from the (113) plane of an xcex1-alumina phase in the measurement of the alumina powder by X-ray diffraction method.
(5) A method for producing a grain-oriented silicon steel sheet not having inorganic mineral films according to any one of the items (1) to (4), further comprising the step of mixing magnesia having a BET specific surface area of 0.5 to 5 m2/g is added to the alumina powder by 5 to 30 weight % relative to the total weight of the alumina powder and the magnesia powder.
(6) A method for producing a grain-oriented silicon steel sheet not having inorganic mineral films according to any one of the items (1) to (5), wherein the average grain size(s) of the alumina powder, and/or the magnesia powder if added, is/are 200 xcexcm or less.